1. Field of the Invention
The present invention relates to the field of fabrication of semiconductor devices. More specifically, the invention relates to the fabrication of epitaxial base bipolar semiconductor devices.
2. Background Art
In an epitaxial base bipolar transistor, a thin layer of silicon, or silicon-germanium, is grown as the base of a bipolar transistor on a silicon wafer. The epitaxial base bipolar transistor has significant advantages in speed, frequency response, and gain when compared to a conventional implanted base silicon bipolar transistor. Speed and frequency response can be compared by the cutoff frequency which, simply stated, is the frequency where the gain of a transistor is drastically reduced. More technically, the current gain approaches a value of one as the frequency of operation of the transistor approaches the cutoff frequency. Cutoff frequencies in excess of 100 GHz have been achieved for silicon-germanium epitaxial base bipolar transistors, which are comparable to those achieved in more expensive GaAs devices. Previously, implanted base silicon bipolar transistors have not been competitive for use where very high speed and frequency response are required.
The higher gain, speeds, and frequency response of the epitaxial base bipolar transistor have been achieved as a result of certain advantages not possible in implanted base silicon bipolar transistors, in particular, the ability to incorporate silicon-germanium layers to form a heterojunction bipolar transistor (xe2x80x9cHBTxe2x80x9d). Silicon-germanium may be epitaxially grown on silicon wafers using conventional silicon processing and tools, and allows one to engineer device properties such as the band gap, energy band structure, and mobilities. For example, it is known in the art that grading the concentration of germanium in the silicon-germanium base builds into the HBT device an electric field, which accelerates the carriers across the base, thereby increasing the speed of the HBT device compared to a silicon-only device. One method for fabricating silicon and silicon-germanium devices is by chemical vapor deposition (xe2x80x9cCVDxe2x80x9d). A reduced pressure chemical vapor deposition technique, or RPCVD, used to fabricate the HBT device allows for a controlled grading of germanium concentration across the base layer. As already noted, speeds in the range of approximately 100 GHz have been demonstrated for silicon-germanium devices, such as the HBT.
Because the benefits of a high gain and high speed silicon-germanium HBT device can be either partially or completely negated by a high base contact resistance, it is important that the resistance of the base contact be kept low. In addition to providing low resistance in the base contact, the geometry of the base region may necessitate providing a low resistance electrical pathway through a portion of the base itself between the base contact and the base-emitter junction. In order to provide lower resistance from the base contact to the base-emitter junction, the extrinsic base region is heavily doped by implantation (or extrinsic doping). The heavily doped extrinsic base region has a reduced resistance.
The region in the base between the edge of the heavily doped extrinsic base region and the edge of the base-emitter junction is referred to as the link base region. The link base region adds a significant amount of resistance between the base contact and the base-emitter junction. It is, therefore, important for the reasons stated above that resistance of the link region also be kept low. The resistance of the link base region is affected by the distance across the link base region from the heavily doped extrinsic base region to the edge of the base-emitter junction. Since the base-emitter junction is substantially coterminous with an xe2x80x9cintrinsic base region,xe2x80x9d the link base region spans a distance between the intrinsic base region and the extrinsic base region. In other words, the link base region xe2x80x9clinksxe2x80x9d the extrinsic base region to the intrinsic base region. The distance across the link base region from the heavily doped extrinsic base region to the intrinsic base region must be no smaller than a certain minimum separation distance in order to provide separation between the heavily doped region of the extrinsic base and the heavily doped region of the emitter near the base-emitter junction.
The link base region itself is relatively lightly doped. If the separation between the heavily doped region of the extrinsic base and the heavily doped region of the emitter near the base-emitter junction is not greater than a minimum separation distance, the two heavily doped regions can form a high field junction and increase the leakage current between the emitter and the base, thereby degrading the performance characteristics of the HBT device. Depending on the alignment of the sequence of steps in the fabrication process used to form the intrinsic base region, to form the base-emitter junction, and to implant the heavily doped extrinsic base region, the distance across the link base region to the intrinsic base region can vary, often unpredictably. With perfect alignment of the sequence of steps in the fabrication process, the distance across the link base region can be minimized to the minimum separation distance just discussed. In that case, the link base resistance would also be minimized. Accounting for the misalignment of the sequence of steps in the fabrication process, however, forces the fabrication of a much greater distance across the link base region than the minimum separation distance. Thus, the link base resistance is greater than the minimum possible link base resistance.
It is important to provide a low resistance in the base contact, the heavily doped extrinsic base region, and the link base region in order to allow the formation of an optimum low-resistance conduction path from the base contact to the intrinsic base region of the HBT or other similar device such as a conventional bipolar transistor. Because the resistances of the base contact, the heavily doped extrinsic base region, and the link base region are in series, the reduction of any one of them will provide an improvement in the resistance of the conduction path from the base contact to the base of the HBT or other similar device.
Thus, there is need in the art to reduce the link base resistance. There is further need in the art to reduce the link base resistance without regard to the alignment of the sequence of steps in the fabrication process. There is also need in the art to reduce the link base resistance without creating additional steps in the sequence of steps in the fabrication process.
The present invention is directed to method for reducing base resistance and related structure. The invention overcomes the need in the art to reduce the link base resistance. The invention also reduces the link base resistance without regard to the alignment of the sequence of steps in the fabrication process. Further, the invention reduces the link base resistance without creating additional steps in the fabrication process.
According to the invention, a dopant spike region is formed in a link base region, which connects an intrinsic base region to an extrinsic base region. For example, the intrinsic base region can be the region in which the base-emitter junction is formed in a silicon-germanium heterojunction bipolar transistor, and the extrinsic base region can be the external portion of the base of the same transistor to which external electrical contact is made. The dopant spike can be an increased concentration of boron dopant.
A diffusion blocking segment is then fabricated on top of the link base region in order to prevent diffusion of the dopant spike out of the link base region. For example, the diffusion blocking segment can be formed from silicon-oxide. Thus, link base resistance is reduced, for example, by the higher concentration of boron dopant in the dopant spike region causing the link base resistance to be lower than the intrinsic base resistance. Moreover, a structure comprising a base region with reduced link base resistance can be fabricated according to the method of the present invention.